1. Field of the Invention
The present invention relates to a dirt filter for semifluid thermoplastic media with a filter chamber hollowed out from a housing and which issues with a circular cross-section into the filter chamber coaxially to a flow axis, with a drain line hollowed out from the casing and which with a circular cross-section emanates from the filter chamber on the side opposite to the feed line inlet and coaxial to the flow axis, the filer chamber and barrier member being constructed in axially symmetrical manner with respect to the flow axis and smooth and the internal diameter of the filter chamber, measured at right angles to the flow axis, being at all points larger than the external diameter of the filter member measured at the same axial height and the difference between said two diameters initially decreasing and then increasing in the flow direction.
2. Brief Description of the Background of the Invention Including Prior Art
In the injection molding of thermoplastic material, the medium which becomes semifluid on heating is fed to an injection nozzle and from the latter is supplied to a fixed injection mold. Impurities and contaminants within the medium, e.g. metal fragments, can block the injection nozzle and can lead to faulty moldings.
In the case of dirt filters of the aforementioned type, the dirt particles jam in the inflow side part of the barrier member between the latter and the wall of the filter chamber and are consequently held back by the same.
In a known dirt filter of the aforementioned type, the barrier member is a ball and the filter chamber is roughly shaped like an ellipsoid, whose larger axis extends in the direction of the flow axis. As a result of the necessary deflection, the ball configuration of the barrier member offers the inflowing thermoplastic medium an undesired resistance. In this known filter, the filtering action is not variable, not even through the replacement of the barrier ball by an e.g. smaller ball, because the latter does not then assume the position necessary for a correct filtering within the filter chamber.
Orsini teaches in U.S. Pat. No. 2,636,218 an extrusion unit for molding plastic materials. It appears that there is no suggestion in this reference as to an in situ cleaning of the body 11. European Patent Application 0 010 338 to Gerrit Jan Ter Beek teaches a filter for filtering viscous liquids and particularly molten synthetic resins. The material is passed and filtered through a slit defined by a chamber and a smooth strapped revoluntionary body. The reference apparently does not suggest moving the revoluntionary body from the outside.